Third Generation Partnership Project (3GPP) standardization technical specification provides carrier aggregation (CA) technique using multiple component carriers (CCs) simultaneously for radio communications.
In carrier aggregation up to Long Term Evolution (LTE) release 10, component carriers from the same evolved node B (eNB) are aggregated to simultaneously conduct radio communications, thereby improving the throughput.
In Release 12, “dual connectivity” is discussed, which technique extends the carrier aggregation technology to inter-eNB or inter-site carrier aggregation to perform simultaneous radio communications using component carriers from different eNBs. See, for example, non-Patent Documents 1 and 2 listed below.
“Dual connectivity” may be a technology equivalent to inter-eNB carrier aggregation and with this technology, further improvement of throughput is expected. For example, dual connectivity is efficient tool to achieve as high throughput as that in Release 10 under the situation where all the component carriers cannot be originated from a single eNB.
Technologies on LTE release 10 and beyond are called LTE-advanced. In LTE-advanced, a random access (RA) procedure is started in order to achieve timing synchronization between user equipment (UE) and an eNB. Random access procedure is triggered at, for example, initial access to an eNB, downlink or uplink (DL/UL) data resuming following discontinuous reception (DRX) cycles, handover procedures, or timing alignment on an SCell.
Two types of random access procedures are defined, namely, contention free random access (CFRA) and contention-based random access (CBRA). Contention free random access is a RA procedure performed responsive to an instruction from the network. A dedicated preamble assigned by the network is used to perform CFRA. DL data resuming and handover are included in this type of random access procedure. Contention based random access is a RA procedure performed autonomously by user equipment (UE), or performed under the control of the network when dedicated preambles are insufficient. This type of random access procedure includes initial access procedure and DL data resuming.